The present invention relates to heat resistant maleimide resins and a process for producing the heat resistant resins, which are soluble in low boiling temperature solvents and have an excellent workability.
As materials having a heat resistance of at least heat-resistant class H (180.degree. C.), there have been well known polyimides of dehydration-condensation type and maleimids of amine-curing type. Among them, aromatic bismaleimides of amine-curing type which are free of problems of condensed water, etc. are now becoming the most important materials in the field.
As maleimides of amine-curing type, aminobis maleimides whose main structure is an aromatic bismaleimide are most widely used. The aminobismaleimides are prepolymers comprising aromatic bismaleimides added with aromatic diamines at the double bond of the former. If the aminobismaleimides are heated, complicated cross linkages are formed due to the amine addition and the radical polymerization of the double bonds to yield polymers having quite excellent heat resistance.
However, in case where the aromatic aminobismaleimides or the aromatic bismaleimides are prepared and used in the form of a solution, a high boiling point solvent such as N-methyl-2-pyrrolidone or N,N-dimethylformamide is necessitated and, as a result, problems of workability, etc. is necessary to dissolve both the bismaleimide and the resulting prepolymer (aminobismaleimide).
Further, a high temperature and a long period of time are required for curing the prepolymer by removing the solvent and by effecting chemical reactions.
There have been known various type of maleimide resins disclosed in such as U.S. Pat. No. 3,678,008, U.S. Pat. No. 4,005,154, U.S. Pat. No. 4,064,193, U.S. Pat. No. 3,883,486, U.S. Pat. No. 3,732,188, U.S. Pat. No. 4,131,632, French Patent No. 76 18582 (Publication No. 2,316,267), U.S. Pat. No. 3,878,172, U.S. Pat. No. 4,064,835, British Pat. No. 1,407,511, British Pat. No. 1,457,270 British Pat. No. 1,355,407 (=German Offenlegungsschrift No. 2,230,874) and British Pat. No. 1,395,761 (=German Offenlegungsschrift No. 2,212,661). These prior arts do not disclose diamines represented by the formula: ##STR2##
Diamines of this type are disclosed in such as British Pat. No. 1,030,026, U.S. Pat. No. 4,017,459 and U.S. Pat. No. 3,505,288.
The present invention have found that the above-mentioned aromatic diamines are particularly useful as a diamine component for production of maleimide prepolymers, which are well soluble to low boiling temperature solvents.
According to the present invention there is provided a new maleimide resin obtained by reacting under heating a maleimide compound having the general formula: ##STR3## with an aromatic diamine having the general formula: ##STR4## where A is an organic group having at least two carbon atoms, especially aromatic and/or aliphatic groups of C.sub.2 -C.sub.25, n is an integer of 1 to 5, R.sub.1 through R.sub.4 are the same or different atoms or groups selected from the group of hydrogen, lower alkyl group having carbon atoms of 1 to 8, lower alkoxy group having carbon atoms of 1 to 8, chlorine and bromine, and R.sub.5 and R.sub.6 are the same or different atoms or groups selected from the group of hydrogen, methyl group, ethyl group, trifluoromethyl group, and trichloromethyl group.